The present invention relates to a filtering apparatus for an air treatment apparatus.
Air filters having very high separation percentages (HEPA, ULPA) require, that the integration into the air treatment apparatus or the duct system be of a very high quality. Not desirable air streams, so called by-pass streams, which can pass through the filtering apparatus without being purified, must be reduced as much as possible. Otherwise the filtering function will be inferior despite the fact, that the filter blanks being used have a very advanced quality.
Typical such by-pass paths exist usually between several filter cartridges, which are disposed in a transverse direction in relation to an air stream to be purified, and/or between such filter cartridges and surrounding duct walls and/or via the housing, which often is a double-walled construction, e.g. in an air treatment apparatus.
The foremost reason for this not desirable by-pass leakage is the relatively large decrease in pressure, which is caused by the air passage through the filter cartridges.
The filter cartridges normally comprise folded filter paper with separators, enclosed in a housing made of steal sheet or wood. The dimensions are normally those of an entire module (about 0.6xc3x970.6 m) or a semi-module. Along the periphery of the housing on the air downstream side, which is the xe2x80x9cpurifiedxe2x80x9d side, there is a seal. The tightness is obtained, because the seal of the filter module is pressed against a plane surface in the form of a flange construction in the duct or an surrounding filter box. This is shown in FIG. 1, which also shows, that the above-mentioned by-pass paths remain despite an adequate sealing within the area of said seal.
In certain cases relatively small filter boxes are used, which are connected to an air duct and in which one or two filter cartridges are enclosed. In case just one filter cartridge is used, the tightness problem can be solved by welding a counter flange to the inner side of the box. However, in case several filter cartridges are used, they and the cassettes, which enclose them, respectively must be sealed against each other and/or against the wall of the box. However, such solutions are time-consuming and often expensive and do not always allow a desirable flexibility, when exchanges are carried out.
When filter cartridges are enclosed in air treatment assemblies, a complicated and expensive duct system is avoided, but on the other hand the tightness problems, mentioned above, arise in a plurality of possible places.
The conventional way of enclosing filter modules in air treatment assemblies comprises the step of joining the filter modules, disposed in their sheet cassettes, to a continuous package. In order to meet the tightness requirements every sheet cassette must be sealed with a sealant all around as well as against the inner sides of the assembly. This sealing is a very delicate and time-consuming process, which means, that a secondary sealing always is recommended on the installation site in connection with the starting of the apparatus. The movements of the housing, e.g. due to pressure differences in the assembly, means, that the sealant must have such properties and be applied in such a way, that it is able to absorb movements without loosing its sealing function.
The object of the present invention is to provide a new filter apparatus, which eliminates the above-mentioned drawbacks and which also in other respects further develops the state of the art in this field.